1. Field of the Invention
The present invention relates to a picture signal transmitting system. More particularly, it relates to a picture signal transmitting system in which picture and digital signals sent through coaxial or optical fiber cables from a CATV network center are transmitted to each subscriber via a junction terminal without converting carrier-modulation format of the picture and digital signals.
2. Description of the Related Art
In recent years, a picture signal transmitting system employing a CATV network is widely used for linking between a CATV center and subscribers through coaxial or optical fiber cables and transmitting high-quality picture and digital signals to the subscribers.
FIG. 10 shows a structural diagram showing an outline of a currently used picture signal transmitting system employing a CATV network. In the picture signal transmitting system, a main transmission path is formed of an optical fiber cable 11 and a coaxial cable 12 used both for upward and downward transmission.
Bi-directional amplifiers 13 are positioned at certain intervals on the main transmission path. Further, a branch transmission path 14 is extended from the main transmission path to a subscriber through a tap off 3, i.e., a junction terminal, in order to link to a protector 4 in subscribers' house 2. A home terminal 21, a digital set top 22, a telephone set top box 23 respectively provided in subscribers' house 2 are connected to protector 4.
FIG. 11 is a diagram for explaining frequency allocation of signals in the picture signal transmitting system employing the CATV network shown in FIG. 10. Downward signals flowing from CATV center 1 toward subscribers' house 2, for example, analog picture signals required for 73 channels are allocated in the frequency band 70-550 MHz. Digital picture signals for 33 carriers are transmitted in the frequency band 550-750 MHz.
Further, upward picture signals required for 7 channels flowing from subscribers' house 2 toward CATV center 1 are allocated in the frequency band 10-55 MHz.
In such the currently used picture signals transmitting system using a CATV network, branch transmission path 14 is extended from the main transmission path through tap off 3 to each subscriber, and is linked to protector 4 of the subscriber's house. A distance between tap off 3 and protector 4 of subscribers'house 2 may be allowed to be at most 30 m extent.
In this example, the above-described main transmission path formed of optical fiber cable 11 and coaxial cable 12 is generally extended by using utility poles as props.
Therefore, installation of the picture signal transmitting system using a CATV network to areas where there is no prop requires new props near each subscribers' houses. Further, there is a case where any utility pole can not be provided to keep views.
More particularly, when distributing the CATV picture signals to a gathered residence section, such as an apartment or a condominium, the signal transmission can be easily executed when transmission paths have been wired to each subscriber in the gathered residence section. Otherwise, it is required to connect branch transmission path 14 for dropping signals from tap off 3. In this case, there are problems of increasing the cost and impairing views.
Further, in the bi-directional CATV system, when subscribers live in the gathered residence section, a control signal sent from each subscriber is transmitted toward the CATV center through a mixer so that noise becomes accumulated.